The present invention relates to back-up instruments which deliver a minimum of three critical items of information to the pilots of an aircraft, in the event of a failure of the main instruments, these items being: the speed of the aircraft relative to the air or conventional speed deduced from a measurement of dynamic pressure, the altitude of the aircraft deduced from a measurement of static pressure and the attitude of the aircraft obtained on the basis of information originating from inertial sensors.
The conventional speed is the image of the dynamic pressure on which the lift depends. In particular, it enables the pilot of an aeroplane to decide the instant of take-off and to estimate his safety margin with respect to the risk of stalling or of abrupt loss of lift.
The altitude is used differently depending on the phases of flight. When cruising, the aeroplane must comply with the air corridor allocated to it and, to do so, must maintain a given flight level defined in terms of a standard altitude or pressure altitude. The standard altitude or pressure altitude is a theoretical altitude deduced from the measurement of the static pressure and translated into a theoretical height after making a number of assumptions regarding the physical properties of the atmosphere in which the aircraft is flying. It is very different from the actual altitude relative to the ground or to sea level. The disparities may reach several thousand feet but the indication provided in terms of relative value for one and the same place is very accurate and leads to a very safe definition of the air corridors since all the aircraft use the same model of the atmosphere.
The flight levels or corridors allocated to the aircraft by the air traffic controllers have a height which takes account of the accuracy of measurement of the standard altitude or pressure altitude. For example, above the level 290 corresponding to 29,000 feet, an air corridor has a height of 1000 feet whereas the accuracy of measurement of the standard altitude or pressure altitude is better than 100 feet. The risk of collision is therefore almost zero.
In the event of inadequate visibility, the pilot must be furnished with a vertical reference, given by an attitude indicator, in order to fly the aircraft in complete safety and avoid placing the aircraft into an attitude which is prohibited by the construction specifications of the machine.
These three basic items of information, regarded as critical in the safety sense, are provided to the pilot and to the copilot of an aircraft in a redundant form so that the probability of total loss or of erroneous information is almost zero. For aircraft which are required to transport passengers, it is the rule that the on-board instruments providing this basic information are triplicated. On board there are, almost routinely, three independent sets of instruments: a first for the pilot, a second for the copilot and a third as back-up. The pilot and the copilot are each furnished with their own set of instruments together with so-called main displays based on cathode-ray tubes or liquid crystals whilst the back-up set of instruments is arranged on a central console separating the pilot""s seat from the copilot""s seat.
The back-up instruments need not be quite as accurate as the main instruments intended for the pilot and for the copilot since, normally, they merely serve as reference for testing the proper operation of the main instruments. But they must however exhibit very high operational reliability.
Until very recently, the reliability of the back-up instruments was based on purely mechanical or pneumatic embodiments not requiring recourse to the on-board electrical supply network or to the mount on the outside of the fuselage of measurement probes other than pressure probes, with no moving parts, connected up by pneumatic ducting. Conversely, in order to improve the accuracy of the indications provided, the embodiments of the instruments of the main systems are calling upon an ever larger portion of electronics involving the use of the on-board electrical supply network and upon various measurement probes, including aerodynamic incidence probes, which are mounted on the outside of the fuselage of the aircraft and enclose electromechanical sensors requiring electrical cable wiring. The reliability of the main instrument systems is affected by the reliability of the on-board electrical network and of the non-purely static measurement probes used on the outside of the fuselage. It is also affected by a degree of sensitivity to the radioelectric disruptions of the environment of the aircraft such as lightning strikes due to the presence of the electrical cable wiring connecting the instruments to certain measurement probes placed outside the fuselage.
In view of these considerations, the search for the greatest possible safety of operation has hitherto led to the use as back-up instruments of: a pneumatic altimeter, a mechanical anemometer or xe2x80x9cair speed indicatorxe2x80x9d and an attitude indicator or gyroscopic xe2x80x9cartificial horizonxe2x80x9d.
The current technological advances have now made it possible to attain a level of safety in respect of the electrical supply network of an aircraft such that it can be presumed that an electrical power source will always be available on board for a minimum of instruments and equipment regardless of the failure conditions encountered. This is why the introduction of electronics into the back-up instruments has recently been planned so as to improve their accuracy and reduce their maintenance costs, without thereby affecting their safety of operation. In particular, it is envisaged to replace the three purely mechanical and pneumatic conventional back-up instruments with fully electronic combined instruments which provide on a liquid crystal screen the three critical items of information with regard to the conduct of a flight, namely: the pressure altitude, the conventional speed and the attitude of the aircraft. However, in order to ensure a high level of safety, it still remains necessary to maintain complete independence between the system of back-up instruments and the systems of main instruments for the pilot and the copilot and also to avoid the system of back-up instruments having recourse to measurement probes mounted on the outside of the fuselage of the aircraft which are not purely static or require connection by electrical wiring. This leads to the system of back-up instruments keeping its own measurement portions and its customary measurement probes. Thus, the system of back-up instruments will still possess its own inertial means for determining the attitude of the aircraft relative to the vertical and its own measurement portions linked to pneumatic measurement probes for determining the conventional speed and the pressure altitude.
The inertial means of the system of back-up instruments consist, for example, of a gyroscopic top slaved to the apparent vertical by an erector device which maintains a vertical reference accurate to within a few tenths of a degree, it being possible to synthesize this gyroscopic top by means of inertial sensors of angular velocity and of accelerometers or inclinometers.
The conventional speed measurement portion of the system of back-up instruments deduces, in a standard manner, the conventional speed from the dynamic pressure which is the difference between the total pressure and the static pressure, by implementing St Venant""s law or Rayleigh""s supersonic equation.
The portion for measuring the pressure altitude of the system of back-up instruments deduces, in a standard manner, the pressure altitude from the static pressure, by applying the benchmark relations arising from the Laplace equations for the standard atmosphere.
The measurements of total pressure and static pressure on behalf of the system of back-up instruments are made independently of the main systems of instruments for the pilot and the copilot, the total pressure being measured with the aid of a specific pressure sensor linked to a Pitot tube installed on the fuselage of the aircraft parallel to its longitudinal axis whereas the static pressure is measured with the aid of another specific pressure sensor linked to a particular air intake which is influenced as little as possible by the aerodynamic field of the aircraft.
The total pressure is easy to measure since it corresponds very accurately to the pressure measured at the bottom of a Pitot tube, as soon as this tube is aligned approximately in the direction of the airflow over the fuselage of an aircraft. The accuracy is better than 1% in the subsonic flight domain.
The static pressure around an aircraft is disturbed by the presence of the aircraft itself since the aerodynamic field of the aircraft modifies the pressure around its fuselage. This modification is approximately proportional to the square of the speed of the local flow. To measure the static pressure, two types of air intake are mainly used. Those of one type consist of orifices drilled in the very surface of the fuselage of the aircraft, usually towards the front of the fuselage. Those of the other type form part of a special anenometric antenna or static Pitot probe.
The disparity between the true static pressure and the static pressure given by a static pressure intake on an aircraft is mainly dependent on the location of the intake on the aircraft, on the sideslip, on the Mach number and on the aerodynamic angle of incidence of the aircraft. This disparity is characterized by a coefficient termed the xe2x80x9cstatics coefficientxe2x80x9d, of the same nature as the pressure coefficient employed for the study of the pressure distributions around the profiles.
To reduce the statics disparity, modern aircraft are equipped either with fuselage static intakes with aerodynamic correction of the statics error by means of a particular profiling of the surface of the fuselage in the immediate neighbourhood of the air intake, or with static Pitot probes aerodynamically compensated by a particular profiling of the tube of the probe and by lateral auxiliary orifices.
In the latter case, the probe is defined in such a way as to exhibit a degree of sensitivity to the variations in the local aerodynamic angle of incidence, this sensitivity being calculated in such a way as to compensate for the variations of the local aerodynamic field as seen by the probe at the location adopted on the fuselage or at some other specific spot.
The tailoring of the shape of an aerodynamically compensated static Pitot probe is difficult since the aerodynamic field depends on the flight conditions and, to a lesser extent, on the configuration of the aircraft (position of the control surfaces, of the landing gear etc.). In practice, a static pressure intake on an aircraft always remains prone to a systematic measurement error dependent, in essence, outside of the phases of dynamic flight, on the sideslip of the aircraft, on the Mach number and on the aerodynamic angle of incidence of the aircraft.
It is known practice to combat the influence of the sideslip by mixing the pressure information arising from two static pressure probes arranged on each side, right and left, of the fuselage of the aircraft.
It is also known practice to take account of the Mach number and of the aerodynamic angle of incidence of the aircraft by applying, to the measured value of static pressure, a correction factor of which the value is chosen as a function of the Mach number and of the aerodynamic angle of incidence of the aircraft according to laws defined during the design of the aircraft and its early flight trials.
Hitherto, the correcting of the static pressure measurement as a function of the Mach number and of the aerodynamic angle of incidence of the aircraft has not been done in the system of back-up instruments which are purely mechanical. It has been done solely by the calculators of the pilot""s and copilot""s main systems of instruments which derive the Mach number from the ratio prevailing between the measured total pressure and the measured static pressure and which receive the value of the aerodynamic angle of incidence from one or more incidence probes or vanes placed on the flanks of the aircraft.
The introduction of an electronic portion into the system of back-up instruments for the control of an aircraft makes it possible to correct the static pressure measured as a function of the Mach number and of the aerodynamic angle of incidence of the aircraft for a better assessment of the barometric altitude. However, it requires knowledge of the aerodynamic angle of incidence of the aircraft, which can no longer be obtained from an incidence probe based on the flanks of the aircraft, for safety reasons, this probe possibly being out of use in the event of an avionic failure and possibly even inducing failures in the system of back-up instruments by propagating a lightning strike by virtue of its electrical connection.
The aim of the present invention is to solve this difficulty. Its aim is more particularly to improve the accuracy of the back-up instruments of an aircraft without however increasing the cost thereof or decreasing the safety of operation thereof.
Its subject is combined back-up instruments for aircraft delivering indications regarding the conventional speed, the standard altitude and the attitude of the aircraft and accordingly comprising:
two pneumatic inlets for measuring pressure, one for the total pressure, intended to be connected up to a total pressure probe mounted on the aircraft, and the other for the static pressure, intended to be connected up to a static pressure probe,
an electronic portion for measuring total pressure equipped with an electronic pressure sensor connected up to the pneumatic inlet dedicated to the total pressure,
an electronic portion for measuring static pressure equipped with an electronic pressure sensor connected up to the pneumatic inlet for static pressure,
an electronic portion for inertial measurements, equipped with gyrometric and accelerometric or inclinometric inertial sensors,
an electronic calculator deducing an indication of conventional speed from the total pressure and static pressure information provided by the electronic portions for measuring total pressure and static pressure, an indication of standard altitude from the static pressure information provided by the electronic portion for measuring static pressure and indications regarding the attitude of the aircraft (angle of pitch xcex8 and angle of roll xcfx86) relative to a vertical reference direction from the information provided by the electronic portion for inertial measurements,
optoelectronic means of display of the indications of conventional speed, of standard altitude and of attitude which are provided by the electronic calculator, these combined back-up instruments being characterized in that the electronic calculator furthermore comprises:
electronic means for computing the aerodynamic angle of incidence xcex1 of the aircraft operating on the basis of the indication of pitch attitude (angle of pitch xcex8 of the aircraft), of the indication of conventional speed and of the indication of pressure altitude which are delivered by the electronic calculator, and
electronic means of correcting the static pressure information provided by the electronic portion for measuring static pressure taking into account a correction coefficient dependent on the indication of aerodynamic angle of incidence xcex1 of the aircraft as delivered by the electronic means for computing the aerodynamic angle of incidence.
Advantageously, the calculator of the combined back-up instruments furthermore comprises electronic means for computing the Mach number operating on the basis of the ratio of the total pressure derived from the total pressure information provided by the electronic portion for measuring total pressure to the static pressure derived from the static pressure information provided by the electronic portion for measuring static pressure, the electronic means of correcting the static pressure information provided by the electronic portion for static pressure taking into account a correction coefficient dependent both on the indication of aerodynamic angle of incidence of the aircraft as delivered by the electronic means of computing the aerodynamic angle of incidence a and of the Mach number indication delivered by the electronic means of computing the Mach number.